1. Field of the Invention
The present invention relates to methods and apparatus for detecting marks or indicia formed in substrates. In particular, the present invention relates to methods and apparatus for detecting marks or indicia formed in substrates and covered by one or more material layers.
2. Background of Related Art
Semiconductor devices and the various structures thereof are fabricated on semiconductor substrates, such as wafers and other large-scale substrates that include a layer of semiconductive material, such as silicon, gallium arsenide, or indium phosphide. Typically, large numbers of a single type of semiconductor device are fabricated on a large-scale substrate, such as a wafer. While many types of semiconductor devices are fabricated in large quantities, on a large number of wafers, to maximize the efficiency with which fabrication equipment is utilized, some types of semiconductor devices are fabricated in relatively small quantities, on only one or a few wafers.
In order to maximize the throughput of fabrication equipment during fabrication of semiconductor devices in small quantities, fabrication process steps that are not specific to a certain type of semiconductor device, such as material layer deposition processes, it is desirable to simultaneously perform these processes on a plurality of wafers, regardless of the type of semiconductor devices being fabricated on each wafer. To accomplish this task, each wafer must be separated from the other wafers with which it is grouped prior to device-specific processes (e.g., patterning of material layers), then routed and transferred to a process location appropriate for the type of semiconductor device being fabricated thereon.
The substrates on which semiconductor devices are produced are typically marked with characters or other indicia that identify the types of semiconductor devices being fabricated thereon. These marks are then identified following the performance of fabrication process steps that are not device specific so that the substrates may be routed to locations where appropriate device specific fabrication processes may be performed. Such marks are typically recessed in the substrate or in a material layer over the substrate. For example, these marks may be etched or laser ablated into the substrate or a material layer thereover. Characters (e.g., numbers) may be formed of a group of small holes in a dot-matrix type arrangement.
Conventionally, the marks on substrates have been identified by directing a narrow wavelength range of light, often from a red, green, or amber light-emitting diode (LED) source, toward the substrate at a location where the marks should be located. A camera, such as a charge-coupled device (CCD) type camera, optically analyzes the illuminated marks. The optically analyzed marks are then identified by a computer executing optical character recognition (OCR) or similar software. As a backup, images of the marks may also be visually displayed for human scrutiny.
While such conventional mark reading apparatus are useful for detecting substrate marks that remain substantially uncovered with material or that are covered with very thin layers or with layers of some visually transparent or translucent materials, this type of equipment cannot be used to identify marks that are covered with one or more layers of visibly opaque materials or even with some translucent materials. It is also difficult to identify marks formed in a substrate, such as silicon, when transparent or translucent materials (e.g., silicon oxides) of very similar color are disposed thereover. As a consequence, prior to routing substrates to a device-specific fabrication process location, it is often necessary to remove material layers from the portion of a substrate where marks are believed to be located. These additional material removal processes are, however, somewhat undesirable.
U.S. Pat. No. 4,896,034, issued to Kiriseko on Jan. 23, 1990 (hereinafter “the '034 patent”) discloses a method whereby a bar code formed in a silicon wafer and covered by one or more material layers is identified by directing one or more infrared wavelengths of electromagnetic radiation toward the bar code through the back side of the substrate. As silicon is relatively transparent to infrared wavelengths of electromagnetic radiation, the infrared radiation readily travels therethrough without a significant degree of reflectance or absorption. An infrared radiation detector is positioned so as to detect only the infrared radiation reflected from the recesses of the bar code. The method of the '034 patent can, however, only be employed to read marks or indicia through the back side of the wafer and not from the more readily accessible active surface of the wafer. Furthermore, the '034 patent only teaches a method for reading bar codes, not for identifying other types of marks, such as characters, or other indicia recessed in a substrate or in a layer disposed over the substrate.
The inventor of the subject matter disclosed herein is unaware of any teaching in the art of a method or an apparatus for identifying marks or indicia through one or more opaque or visibly opaque material layers.